The proposed research investigates the structure of the cloned receptor for erythropoietin (EP), the metabolism of the receptor protein, the possible interaction of the receptor protein with additional subunits, the interaction of the EP receptor with the gp 55 glycoprotein of the Friend virus, the distribution of EP receptors in tissues other than erythroid, and signal transduction through changes in phosphorylation following the interaction of EP with the receptor. These studies will provide fundamental data on the mechanisms by which erythropoiesis is regulated and should be useful in understanding the aberrant control which results in anemia and polycythemia as well as ways to correct these disease states. One specific aim is to identify the posttranslational modification of the cloned EP receptor in normal erythroid cells which results in high molecular weight (78k) receptors. Secondly, the metabolism of the receptors for EP will be studied to find the reason that the high molecular weight forms of the receptor are modulated up and down by the absence and presence, respectively, of EP. The interaction of the cloned receptor protein with other proteins will be studies to see if: (1) receptors dimerize or form larger aggregates; (2) receptors interact with a 105 kDa protein which may be a second subunit of the receptor; and (3) the EP receptors of varying molecular weight bind the gp 55 viral envelope proteins of both the anemia and polycythemia strain of Friend virus in either cells where the gp 55 confers EP independent proliferation or cells where the gp 55 has no effect. Cell surface biotinylation will be carried out to establish the ratio of cell surface to intracellular receptors and the structure of cell surface receptors. A fourth specific aim is to study the signal transduction through the EP receptor by quantitating changes in the phosphorylation on tyrosine residues of cellular proteins following the binding of EP to erythroid cells. The effect of EP will be compared in multipotent hematopoietic cells and proerythroblasts which respond exclusively to EP. In addition the erythroid cells from normal humans and patients with polycythemia vera will be compared to see if alterations in protein phosphorylation correlate with the hyperproliferation of erythroid cells in the disease state.